Golf ball

ABSTRACT

A golf ball of the present invention includes a core and a cover located outside the core and having a plurality of dimples on a surface thereof. A bottom of each of the dimples has a curved shape protruding toward an outside of the golf ball, and the protruding portion has a spherical crown shape having a radius of curvature R of 20 to 50 mm. A depth d of a center protruding portion of the bottom of the dimple is a perpendicular distance between a line S connecting both ends of an outer periphery of the dimple and a highest point of the protruding portion. A volume occupation ratio VR of the dimple is less than 0.75. A relationship between Shore D hardness H of a material of the cover and the depth d (unit: mm) satisfies the following Formula 1: 
       ( H −78)/(−300)&gt; d   (Formula 1).

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from Japanese Patent Application No. 2022-97408 filed Jun. 16, 2022, which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a golf ball.

It is well known that when a golf ball is hit, backspin is applied to the golf ball. If too much backspin is applied on a driver shot, the ball tends to pop up. Accordingly, in order to extend a flight distance, it is generally required to reduce the amount of backspin. In order to reduce the amount of backspin, a golf ball which can have an increased frictional force with a face on a driver shot has been proposed.

For example, JP 2017-006555 A discloses a golf ball having a plurality of dimples on a surface thereof, in which: a bottom of each of the dimples has a curved shape protruding toward an outside of the golf ball; and a relationship among a deflection amount H of the golf ball when a predetermined load is applied to the golf ball, a virtual plane area S when no dimple exists on the surface of the golf ball, and a pressurized area PS which is an area of the golf ball contacting a plane when a predetermined load is applied to the golf ball satisfies a predetermined formula.

JP 2017-079905 A discloses a golf ball including a core, a cover, and at least one intermediate layer therebetween, in which: a value obtained by subtracting the surface hardness of an intermediate layer-encased sphere from the surface hardness of the ball, and a value obtained by subtracting the surface hardness of the core from the surface hardness of the intermediate layer-encased sphere are within predetermined ranges; hardnesses at a core center, a position 5 mm from the core center, a position 10 mm from the core center, a position 15 mm from the core center, and a core surface, in core hardness distribution are within predetermined ranges; a value obtained by subtracting the center hardness of the core from the surface hardness of the core is within a predetermined range; and a relationship V/H between a ball initial velocity V and a golf ball deflection amount H when a predetermined load is applied to the golf ball is within a predetermined range.

Furthermore, JP 2017-086579 A discloses a golf ball including a two-layer core including an inner layer and an outer layer, a cover, at least one intermediate layer between the core and the cover, and a coating film layer formed on the surface of the cover. In core hardness distribution, hardness Cc at the center of the inner layer core, hardness C10 at a position 10 mm from the center of the inner layer core, hardness Cs at the surface of the inner layer core, and hardness Css at the surface of the outer layer core satisfy two predetermined formulae. Furthermore, the sphere including the core encased by the intermediate layer has higher surface hardness than that of the ball.

SUMMARY OF THE INVENTION

Any of the documents of JP 2017-006555 A, JP 2017-079905 A, and JP 2017-086579 A disclose that the dimple of which the bottom has a curved shape protruding toward the outside of the golf ball is arranged. The present inventor has found that the dimple may not necessarily contribute to an increase in actual flight distance even if the protruding shape causes an increased frictional force to cause a decreased amount of backspin since the protruding shape is disadvantageous in terms of aerodynamic properties. The frictional force of the dimple having a protruding bottom with respect to the golf club is influenced by the material hardness of the cover, but this is not particularly mentioned in the above documents.

It is therefore an object of the present invention to provide a golf ball in which a bottom of each of dimple has a curved shape protruding toward an outside of the golf ball, which makes it possible to reliably increase a flight distance.

In order to achieve the object, the present invention is a golf ball including a core and a cover located outside the core and having a plurality of dimples on a surface thereof, in which: a bottom of each of the dimples has a portion having a curved shape protruding toward an outside of the golf ball, and the protruding portion has a spherical crown shape having a radius of curvature R of 20 to 50 mm; a depth d of the protruding portion of the bottom of the dimple is a perpendicular distance between a line S connecting both ends of an outer periphery of the dimple and a highest point of the protruding portion; a volume occupation ratio VR of the dimple is less than 0.75; and a relationship between Shore D hardness H of a material of the cover and the depth d (unit: mm) satisfies the following Formula 1:

(H−78)/(−300)>d  (Formula 1).

The Shore D hardness of the material of the cover may be 50 to 60.

The golf ball may further include an intermediate layer between the core and the cover. Shore D hardness of a material of the intermediate layer may be 55 or more.

The Shore D hardness of the material of the intermediate layer may be higher than the Shore D hardness of the material of the cover.

The golf ball may further include a coating layer located outside the cover; the coating layer may contain delustering particles; and average roughness Ra of a surface of the coating layer may be 0.5 to 1.0.

The number of dimples having the protruding portion having the spherical crown shape having the radius of curvature R of 20 to 50 mm may be 50% or more of the total number of dimples on a surface of the cover.

In addition to the dimples having the protruding portion having the spherical crown shape having the radius of curvature R of 20 to 50 mm, dimples having a protruding portion having a spherical crown shape having a radius of curvature R of 12 mm or less may be arranged, and the number of the dimples having the protruding portion having the spherical crown shape having the radius of curvature R of 12 mm or less may be 1 to 10% of the total number of dimples on a surface of the cover.

According to the present invention, the bottom of each of the dimples has a curved shape protruding toward an outside of the golf ball, and when the protruding portion has a spherical crown shape having a radius of curvature R of 20 to 50 mm, the relationship between the depth d of the protruding portion of the bottom of the dimple and the hardness of the material of the cover is specified as in the above-mentioned Formula 1, whereby the contact area of the golf ball with a club face in full shots (shots of a driver to a middle iron) can be increased, which makes it possible to reduce the amount of backspin. Furthermore, the curved protruding shape of the bottom of the dimple is disadvantageous in terms of aerodynamic properties. Only reduction in the amount of backspin makes it impossible to achieve an increase in the flight distance of the golf ball, but the volume occupation ratio VR of the dimple is reduced to be less than 0.75 to heighten the trajectory of the golf ball, whereby the increase in the flight distance can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view which illustrates an embodiment of a golf ball according to the present invention;

FIG. 2 is an enlarged cross-sectional view of one dimple of the golf ball illustrated in FIG. 1 ;

FIG. 3 is a perspective view which illustrates an example of a golf ball of Comparative Example 5;

FIG. 4 is an enlarged cross-sectional view of one dimple of the golf ball illustrated in FIG. 3 ;

FIG. 5 is a photograph which illustrates dimples of a golf ball of Example 1; and

FIG. 6 is a photograph which illustrates dimples of a golf ball of Comparative Example 5.

DESCRIPTION OF PREFERRED EMBODIMENTS

An embodiment of a golf ball according to the present invention will be described below with reference to attached drawings. However, the present invention is not limited thereby.

The golf ball according to the present invention includes, as an embodiment thereof, a core (not shown) and a cover located outside the core and having a plurality of dimples 10 on a surface thereof, as illustrated in FIG. 1 . A portion of the surface of the golf ball 1 located among the plurality of dimples 10 is usually referred to as a land portion 20. The land portion 20 constitutes the spherical surface of the golf ball 1. Accordingly, the land portion 20 has a constant curvature surface.

The planar shape of the dimple 10 formed on the surface of the golf ball 1 (i.e., the shape recognized when an outer periphery 12 of the dimple 10 or a boundary between the dimple 10 and the land portion 20 is viewed from immediately above the dimple) may be circular, polygonal (for example, regular hexagonal), and noncircular, and the like. In the present embodiment, the planar shape is polygonal (substantially regular hexagonal). The diameter of the circular dimple and the diameter of the circumscribed circle of the polygonal dimple are preferably in a range of 2 to 5 mm. It is not necessary for all the dimples formed on the surface of the golf ball to have the same diameter. The diameters of the dimples may differ as long as the diameters are in a range of 2 to 5 mm. For example, it is preferable that at least three types of dimples with different sizes are arranged. This makes it possible to uniformly arrange the dimples on the spherical surface of the golf ball without a gap.

The dimple 10 of the present embodiment has a shape in which a part of the bottom thereof is curved so as to protrude toward an outside of the ball. FIG. 2 illustrates a cross-sectional view of the dimple 10 along the diameter thereof. As illustrated in FIG. 2 , the dimple 10 has a bottom 14 with a curved shape shaped from one end to the other end of the outer periphery 12. The bottom 14 includes a portion with a curved shape protruding toward the outside of the ball in a center region, i.e., a center protruding portion 15, and a portion with a curved shape recessed from the outside of the ball in a ring-like region in its outer periphery.

This center protruding portion 15 has a spherical crown shape (a surface shape of a portion obtained by cutting a sphere with one plane) at its apex, and the spherical crown shape has a radius of curvature R of 20 to 50 mm. When the radius of curvature R of the spherical crown shape of the center protruding portion 15 is more than 50 mm, a center part of the center protruding portion 15 cannot sufficiently contact a club face in full shots in the same manner as in the case where an apex of a center protruding portion 35 as shown in FIG. 3 has a flat shape even if a depth d of the center protruding portion 15, to be described later, is a predetermined depth. In addition, when the radius of curvature R is less than 20 mm, a portion other than the center part of the center protruding portion 15 cannot sufficiently contact the club face. Thus, the center protruding portion 15 has a spherical crown shape having a radius of curvature R of 20 to 50 mm to allow the entire center protruding portion 15 to sufficiently contact the club face in full shots, whereby the amount of backspin of the golf ball can be reduced to extend a flight distance. The upper limit of the radius of curvature R is preferably 45 mm or less, and more preferably 40 mm or less. The lower limit of the radius of curvature is preferably 25 mm or more, and more preferably 30 mm or more.

The diameter of the region of the spherical crown shape of the center protruding portion 15, i.e., a distance W between both ends 17 is preferably in a range of 35 to 65, more preferably in a range of 40 to 60, and still more preferably in a range of 45 to 55, with a distance between the outer periphery 12 and a center point 16 of the dimple as 100.

A depth d of the center protruding portion 15 of the dimple 10 is a perpendicular distance between a line S connecting both ends of the outer periphery 12 of the dimple and a highest point of the center protruding portion 15 (center point 16). A relationship between the depth d (unit: mm) of the center protruding portion 15 and Shore D hardness H of a material which forms a cover to be described later satisfies the following Formula 1:

(H−78)/(−300)>d  (Formula 1).

That is, if the depth d is greater than the value of (H−78)/(−300), the contact area of the golf ball with a face of the golf club is not sufficient, whereby a frictional force cannot be increased. The value of (H−78)/(−300)−d is preferably 0.002 or more, and more preferably 0.005 or more. The depth d providing such a value makes it possible to reliably provide an excellent frictional force depending on the material hardness of the cover. The upper limit of the value of (H−78)/(−300)−d is not particularly limited, and is preferably 0.030 or less, and more preferably 0.025 or less.

The bottom has a curved shape protruding toward the outside of the golf ball in the land portion 20 from a deepest point 18 located on both sides of the center protruding portion 15 so that the depth thereof becomes the largest at the deepest point 18. The location of the deepest point 18 on the plane is preferably in a range of 20 to 45, more preferably in a range of 25 to 40, and still more preferably in a range of 30 to 35, with a distance between the outer periphery 12 and a center point 16 of the dimple as 100.

The depth D of the dimple 10 differs according to the depth d of the center protruding portion 15. For example, the depth D of the dimple 10 is greater than the depth d of the center protruding portion 15 preferably by 0.025 mm or more, and more preferably by 0.030 mm or more. The upper limit of the depth D of the dimple 10 is not particularly limited, and is less than the depth d of the center protruding portion 15 preferably by 0.200 mm or less, and more preferably by 0.150 mm or less.

It is not necessary for all the dimples formed on the surface of the golf ball to have the center protruding portion having the spherical crown shape having the radius of curvature R of 20 to 50 mm as described above. Preferably 50% or more, more preferably 70% or more, still more preferably 80% or more, and most preferably 90% or more of all the dimples have a predetermined center protruding portion. Of course, all the dimples may have the center protruding portion. From the viewpoint of exerting excellent aerodynamic isotropy and air resistance, it is preferable that the dimples having the center protruding portion described above be uniformly arranged over the entire surface of the golf ball.

It is not necessary for all the dimples having the center protruding portion having the spherical crown shape to have the same radius of curvature R. The dimples may have different radii of curvature R in a range of 20 to 50 mm. For example, 1 to 10%, 20 to 40%, and 30 to 50% of all the dimples formed on the surface of the golf ball may respectively have a radius of curvature R of 20 mm or more and less than 30 mm, 30 mm or more and less than 40 mm, and 40 mm or more and 50 mm or less. In order to exert the same effect as the above also for dimples having a small diameter when dimples having different diameters are arranged on the surface of the golf ball, a center protruding portion having a spherical crown shape having a radius of curvature R which is less than the above, for example, 2 to 12 mm can be formed on the bottom of this small dimple. That is, dimples having a center protruding portion having a spherical crown shape having a radius of curvature R of 20 to 50 mm and dimples having a center protruding portion having a spherical crown shape having a radius of curvature R of 2 to 12 mm may be arranged on the surface of the golf ball. For example, 1 to 10% of all the dimples formed on the surface of the golf ball may have a radius of curvature R of 2 to 12 mm.

The upper limit of the total number of the dimples is, but is not limited to, preferably 500 or less, and more preferably 450 or less. The lower limit of the total number of the dimples is, but is not limited to, preferably 250 or more, and more preferably 300 or more.

A volume occupation ratio VR of the dimples (i.e., a ratio of the total volume of the dimples formed in a portion downward from the plane surrounded by the edge of the dimple in relation to a virtual spherical volume of the golf ball obtained supposing that no dimple exists on the surface of the golf ball) is less than 0.75%. The dimples having the center-protruding shape described above are disadvantageous in terms of aerodynamic properties. Therefore, the volume occupation ratio VR of the dimple is set to be less than 0.75% to heighten the trajectory of the golf ball, whereby the flight distance can be increased. The volume occupation ratio VR of the dimple is preferably 0.73% or less, and more preferably 0.70% or less. The lower limit of the volume occupation ratio VR of the dimple is not particularly limited, and is, for example, preferably 0.65% or more, and more preferably 0.68% or more.

A surface occupation ratio SR of the dimples (i.e., a ratio of the total area occupied by the dimples to the entire surface area of a virtual spherical surface of the golf ball obtained by supposing that no dimple exists on the surface of the golf ball) is preferably 70% or more, more preferably 75% or more, and still more preferably 80% or more. The upper limit of the surface occupation ratio SR of the dimples is not particularly limited, and is preferably 99% or less.

A material which forms the cover includes an ionomer resin, a polyurethane-based thermoplastic elastomer, a thermosetting polyurethane, and a mixture thereof, but the material is not limited thereto. In the cover, the abovementioned main component can be blended with other thermoplastic elastomers, polyisocyanate compounds, fatty acids or derivatives thereof, basic inorganic metal compounds, and fillers and the like.

The Shore D hardness H of the material which forms the cover satisfies Formula 1 as described above. Therefore, the Shore D hardness H of the material which forms the cover is dependent on the depth d of the center protruding portion of the dimple, and is, for example, preferably 50 or more, and more preferably 53 or more. The Shore D hardness H of the material which forms the cover is preferably 65 or less, more preferably 62 or less, and still more preferably 60 or less. Such numerical value ranges can provide an appropriate amount of spin in shots of a driver to a middle iron.

The lower limit of the thickness of the cover is preferably 0.2 mm or more, and more preferably 0.4 mm or more, but the thickness of the cover is not limited thereto. The upper limit of the thickness of the cover is preferably 4 mm or less, more preferably 3 mm or less, and still more preferably 2 mm or less.

The core can be formed from a rubber composition containing rubber as a main component. As this rubber (base rubber) serving as the main component, synthetic rubber and natural rubber can be widely used; and examples of the rubber which can be used include polybutadiene rubber (BR), styrene butadiene rubber (SBR), natural rubber (NR), polyisoprene rubber (IR), polyurethane rubber (PU), butyl rubber (IIR), vinyl polybutadiene rubber (VBR), ethylene propylene rubber (EPDM), nitrile rubber (NBR) and silicone rubber, but the rubber is not limited thereto. As the polybutadiene rubber (BR), for example, 1,2-polybutadiene or cis-1,4-polybutadiene or the like can be used.

In the core, the rubber composition can be optionally blended with, for example, a co-crosslinker, a crosslinking initiator, a filler, an anti-aging agent, an isomerizing agent, a peptizing agent, sulfur and an organic sulfur compound, in addition to the abovementioned base rubber. In place of the rubber, a resin may be used as the main component, and for example, a thermoplastic elastomer, an ionomer resin or a mixture thereof can also be used.

Preferable examples of the co-crosslinker to be used include α,β-unsaturated carboxylic acid or a metal salt thereof, but the co-crosslinker is not limited thereto. Examples of the α,β-unsaturated carboxylic acid or the metal salt thereof include: acrylic acid and methacrylic acid; and zinc salts, magnesium salts and calcium salts thereof. The blending ratio of the co-crosslinker is, but is not limited to, for example, preferably approximately 5 parts by weight or more, and more preferably approximately 10 parts by weight or more, with respect to 100 parts by weight of the base rubber. The blending ratio of the co-crosslinker is preferably approximately 70 parts by weight or less, and more preferably approximately 50 parts by weight or less.

As the crosslinking initiator, an organic peroxide is preferably used, and examples thereof include dicumyl peroxide, t-butyl peroxybenzoate, di-t-butyl peroxide, and 1,1-bis(t-butylperoxy)3,3,5-trimethylcyclohexane, but the crosslinking initiator is not limited thereto. The blending ratio of the crosslinking initiator is, but is not limited to, preferably approximately 0.10 parts by weight or more, more preferably approximately 0.15 parts by weight or more, and still more preferably approximately 0.30 parts by weight or more, with respect to 100 parts by weight of the base rubber. The blending ratio of the crosslinking initiator is preferably approximately 8 parts by weight or less, and more preferably approximately 6 parts by weight or less.

Examples of the filler which can be used include silver, gold, cobalt, chromium, copper, iron, germanium, manganese, molybdenum, nickel, lead, platinum, tin, titanium, tungsten, zinc, zirconium, barium sulfate, zinc oxide and manganese oxide, but the filler is not limited thereto. The filler is preferably in the form of a powder. The blending ratio of the filler is, but is not limited to, for example, preferably approximately 1 part by weight or more, more preferably approximately 2 parts by weight or more, and still more preferably approximately 3 parts by weight or more, with respect to 100 parts by weight of the base rubber. The blending ratio of the filler is preferably approximately 100 parts by weight or less, more preferably approximately 80 parts by weight or less, and still more preferably approximately 70 parts by weight or less.

Examples of the anti-aging agent which can be used include commercialized products such as NOCRAC NS-6 (manufactured by Ouchi Shinko Chemical Industrial Co., Ltd.), but the anti-aging agent is not limited thereto. The blending ratio of the anti-aging agent is, but is not limited to, preferably approximately 0.1 parts by weight or more, and more preferably approximately 0.15 parts by weight or more, with respect to 100 parts by weight of the base rubber. The blending ratio of the anti-aging agent is preferably approximately 1.0 part by mass or less, and more preferably approximately 0.7 parts by mass or less.

The resilience of the core 40 can be improved by the addition of an organic sulfur compound (peptizer). The organic sulfur compound is selected from thiophenols, thiocarboxylic acids, and metal salts thereof. Examples of the thiophenols and the thiocarboxylic acids include thiophenols such as pentachlorothiophenol, 4-t-butyl-o-thiophenol, 4-t-butylthiophenol and 2-benzamidothiophenol, and thiocarboxylic acids such as thiobenzoic acid. As the metal salts thereof, zinc salts and the like are preferable. The blending ratio of the organic sulfur compound is preferably approximately 0.5 parts by weight or more, and more preferably approximately 1 part by weight or more, with respect to 100 parts by weight of the base rubber, but the ratio is not limited thereto. The blending ratio of the organic sulfur compound is preferably approximately 3 parts by weight or less, and more preferably approximately 2 parts by weight or less.

The upper limit of the Shore D hardness of the material which forms the core is preferably 60 or less, more preferably 50 or less, and still more preferably 40 or less. In addition, the lower limit of the Shore D hardness of the material which forms the core is preferably 20 or more, and more preferably 30 or more, but the lower limit is not limited thereto. With the material hardness of the core in such a range, the feeling of hitting the golf ball can be improved.

The lower limit of the thickness of the core may be 4.5 mm or more in order to impart a predetermined repulsive force to the golf ball, and is more preferably 10 mm or more. In addition, the upper limit of the thickness of the core is, but is not limited to, preferably 25 mm or less, and more preferably 20 mm or less. The core is not limited to a core formed of a single layer, and for example, the core may be formed of a plurality of layers. In this case, it is preferable that the hardness of each layer of the core is controlled so as to increase from the inside to the outside of the golf ball.

An intermediate layer (not shown) may be optionally provided between the core and the cover. The intermediate layer is provided, which can provide an appropriate amount of spin in shots of a driver to a middle iron.

A material which is preferably used as the main material of the intermediate layer is the following heated mixture, but the material is not limited thereto. This material is used for the intermediate layer, whereby the amount of spin can be decreased at the time of hitting, and a greater flight distance can be obtained. The mixture contains: a base resin in which (a) a binary random copolymer of olefin-unsaturated carboxylic acid, and/or a metal ion neutralized product of a binary random copolymer of olefin-unsaturated carboxylic acid, and (b) a ternary random copolymer of olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester, and/or a metal ion neutralized product of a ternary random copolymer of olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester are blended so that the weight ratio of 100:0 to 0:100 is achieved; (e) a non-ionomeric thermoplastic elastomer which is blended so that the weight ratio of 100:0 to 50:50 is achieved with respect to the base resin; (c) 5 to 150 parts by weight of a fatty acid having a molecular weight of 228 to 1500 and/or a derivative thereof, with respect to 100 parts by weight of a resin component containing the base resin and the component (e); and (d) 0.1 to 17 parts by weight of a basic inorganic metal compound which can neutralize an unneutralized acid group in the base resin and the component (c).

Here, the “main material” means a material which accounts for 50% by weight or more, preferably 60% by weight or more, and still more preferably 70% by weight or more, of the total weight of the intermediate layer.

The Shore D hardness of the material which forms the intermediate layer is preferably 55 or more, and more preferably 57 or more. The Shore D hardness of the material which forms the intermediate layer is preferably higher than the Shore D hardness of the material which forms the cover. This can provide an appropriate amount of spin in shots of a driver to a middle iron. The upper limit of the Shore D hardness of the material which forms the intermediate layer is preferably 65 or less, and more preferably 63 or less, but the upper limit is not limited thereto.

The thickness of the intermediate layer is, but is not limited to, preferably 0.5 mm or more, and more preferably 1 mm or more. The thickness of the intermediate layer 20 is preferably 10 mm or less, more preferably 5 mm or less, and still more preferably 3 mm or less.

On the surface of the cover, a coating layer (or also referred to as paint layer) (not shown) may be optionally provided. The coating layer is formed of a coating material composition. The coating material composition may contain delustering particles. The coating material composition is not particularly limited, and for example, a urethane-based coating material is preferably used. Given the need to be capable of enduring the severe use environment of a golf ball, a two-part curable urethane coating material is preferable, with the use of a non-yellowing urethane coating material being particularly preferable.

In the case of the two-part curable urethane coating material, it is preferable to use, as the base resin, various polyols such as saturated polyester polyols, acrylic polyols and polycarbonate polyols. It is preferable to use, as an isocyanate which is a curing agent, a non-yellowing polyisocyanate, examples of which include hexamethylene diisocyanate, isophorone diisocyanate, and adducts, biurets, isocyanurates, or mixtures thereof, of hydrogenated xylylene diisocyanate.

Examples of the delustering particles include silica particles, melamine particles and acrylic particles. Specific examples include silica particles, polymethyl methacrylate particles, polybutyl methacrylate particles, polystyrene particles and polybutyl acrylate particles. Either organic particles or inorganic particles may be used, with the use of silica particles being particularly suitable.

If such delustering particles are contained in the coating layer, the aerodynamic performance of the golf ball is disadvantageous, but in the present embodiment, the volume occupation ratio VR of the dimple is less than 0.75, to heighten the trajectory, whereby the flight distance can be maintained. When the delustering particles are contained in the coating layer, the volume occupation ratio VR of the dimple may be less than 0.70. Thereby, the trajectory of the golf ball is further heightened, whereby the flight distance can be increased.

In terms of the light-quenching properties and the coating properties, the delustering particles have a BET specific surface area, which is preferably 200 to 400 m²/g, and more preferably 250 to 350 m²/g. In terms of the spin performance and the light-quenching properties, the delustering particles have an average primary particle size which is preferably 1.0 to 3.0 μm, and more preferably 2.0 to 2.8 μm. When the average primary particle size is more than 3.0 μm, the ball surface becomes rough, which may have an adverse effect on the spin performance of the golf ball, lowering the spin performance. In addition, when the average primary particle size is too small, the light-quenching effect may diminish.

The blending amount of the delustering particles per 100 parts by mass of the base resin (the total amount of resin components and solvent) in the coating material composition of the coating layer may be set to be preferably 5 to 10 parts by mass. When this blending amount is too high, the viscosity of the coating material composition rises and the coating operation tends to be poor. When it is too low, the light-quenching effect may diminish. The coating layer has average surface roughness Ra which, from the standpoint of both the amount of spin of the ball on approach shots and the light-quenching properties, is suitably 0.5 to 1.0. The surface roughness Ra of the coating film means the arithmetic average roughness in JIS B0601 (1994).

EXAMPLES

A golf ball having a configuration shown in Example 1 of Table 1 was produced. Table 3 shows the blending of a core. Table 4 shows the blendings of an intermediate layer and cover. Table 5 shows the blending of a paint layer. All units are parts by weight. The dimples were arranged to have a pattern illustrated in FIG. 1 . The bottom of the dimple had a curved shape protruding toward an outside of the golf ball in its center range as illustrated in FIG. 2 . The center protruding portion had a spherical crown shape having a radius of curvature R in a range of 25 to 45 mm. The more detailed specification of the dimple is shown in Table 6.

TABLE 1 Examples 1 2 3 4 5 6 Intermediate Blending A A A A A B layer Material hardness 57 57 57 57 57 51 Cover Blending C D E C C C Material hardness H 55 59 62 55 55 55 Paint layer Blending F F F G G F Delustering particles Absent Absent Absent Present Present Absent Dimples Number 326 326 326 326 326 326 VR 0.72 0.70 0.68 0.68 0.72 0.70 Protruding Number 318 318 318 318 318 318 Shape of Spherical Spherical Spherical Spherical Spherical Spherical apex crown crown crown crown crown crown Depth d [mm] 0.065 0.055 0.048 0.053 0.065 0.065 Value of (H-78)/(−300) 0.077 0.063 0.053 0.077 0.077 0.077 Value of (H-78)/(−300)-d 0.012 0.008 0.005 0.024 0.012 0.012 Evaluation of Formula 1 Satisfied Satisfied Satisfied Satisfied Satisfied Satisfied Amount of spin (driver) [rpm] 2692 2676 2661 2688 2685 2696 Evaluation Good Good Good Good Good Good Flight distance (driver) [m] 233.6 233.8 234.2 232.8 231.8 233.3 Evaluation Good Good Good Good Poor Good Amount of spin (middle iron) [rpm] 5084 5211 5287 5123 5142 5578 Evaluation Good Good Good Good Good Poor

TABLE 2 Comparative Examples 1 2 3 4 5 Intermediate Blending A A A A A layer Material hardness 57 57 57 57 57 Cover Blending C D E C C Material hardness H 55 59 62 55 55 Paint layer Blending F F F F F Delustering particles Absent Absent Absent Absent Absent Dimples Number 326 326 326 326 326 VR 0.80 0.72 0.71 0.78 0.70 Protruding Number 318 318 318 318 318 Shape of Spherical Spherical Spherical Spherical Flat apex crown crown crown crown Depth d 0.080 0.065 0.060 0.075 0.065 [mm] Value of (H-78)/(−300) 0.077 0.063 0.053 0.077 0.077 Value of (H-78)/(−300)-d −0.003 −0.002 −0.007 0.002 0.012 Evaluation of Formula 1 Not Not Not Satisfied Satisfied satisfied satisfied satisfied Amount of spin (driver) [rpm] 2764 2753 2748 2702 2722 Evaluation Bad Bad Poor Good Poor Flight distance (driver) [m] 231.1 230.8 231.5 229.4 231.3 Evaluation Bad Bad Bad Bad Bad Amount of spin (middle iron) [rpm] 5215 5344 5432 5082 5102 Evaluation Poor Poor Poor Good Good

TABLE 3 Polybutadiene 100 Zinc acrylate 21 Organic peroxide A 0.3 Organic peroxide B 0.3 Anti-aging agent 0.1 Zinc oxide 29.4 Zinc pentachlorothiophenol 0.6

Polybutadiene in Table 3 is “BRO1” (trade name) manufactured by JSR Corporation. Zinc acrylate is manufactured by NIPPON SHOKUBAI CO., LTD. An organic peroxide A is “PERCUMYL D” (trade name) manufactured by NOF CORPORATION. An organic peroxide B is “PEROXA 40” (trade name) manufactured by NOF CORPORATION. An anti-aging agent is 2,2-methylenebis(4-methyl-6-butylphenol) with “Nocrac NS-6” (trade name) manufactured by OUCHI SHINKO CHEMICAL INDUSTRIAL CO., LTD. Zinc oxide is “ZINC OXIDE 3 TYPES” (trade name) manufactured by SAKAI CHEMICAL INDUSTRY CO., LTD. Zinc pentachlorothiophenol is manufactured by Zhejiang Cho & Fu Chemical Co., Ltd.

TABLE 4 A B C D E HPF1000 56 100 — — — Himilan 1605 44 — 50 50 50 AM7329 — — — 15 50 Surlyn 9320 — — 50 35 — Titanium oxide — — 4 4 4

“HPF1000” in Table 4 is an ionomer resin manufactured by THE DOW CHEMICAL COMPANY “Himilan 1605” is an ionomer resin manufactured by DuPont-Mitsui Polychemicals Co., Ltd. “AM7329” is an ionomer resin manufactured by DuPont-Mitsui Polychemicals Co., Ltd. “Surlyn 9320” is an ionomer resin manufactured by THE DOW CHEMICAL COMPANY.

TABLE 5 F G Base resin Polyol 29.84 29.84 Delustering particles 0 9 Solvent 70.16 70.16 Curing agent Isocyanate 42 42 Solvent 58 58

As a “polyol” of a base resin in Table 5, a polyester polyol synthesized by the following method was used. First, a reactor equipped with a reflux condenser, a dropping funnel, a gas inlet and a thermometer was charged with 140 parts by mass of trimethylolpropane, 95 parts by mass of ethylene glycol, 157 parts by mass of adipic acid and 58 parts by mass of 1,4-cyclohexanedimethanol, followed by raising the temperature to 200 to 240° C. while stirring, for heating (reacting) for 5 hours. Then, a polyester polyol was obtained, which had an acid value of 4, a hydroxyl value of 170 and a weight-average molecular weight (Mw) of 28,000. As “delustering particles”, “Finesil X-35” manufactured by Maruo Calcium Co., Ltd., was used.

As an “isocyanate” for a curing agent, Duranate TPA-100 (trade name) manufactured by Asahi Kasei Corporation as a nurate body (isocyanurate body) of hexamethylene diisocyanate (HMIDI) (NCO content: 23.1%, non-volatile content: 100%) was used. Butyl acetate was used as a solvent for both a base resin and a curing agent. The two-part curable urethane coating material having the above blending was applied with an air spray gun onto the surface of a cover on which dimples had been formed, to form a paint layer.

TABLE 6 Shape of Radius of Ratio of Type of Shape of apex of curvature dimple dimple bottom protruding portion R (mm) (%) A Center protruding Spherical crown shape 10 3.7 B Center protruding Spherical crown shape 25 5.5 C Center protruding Spherical crown shape 30 9.8 D Center protruding Spherical crown shape 35 29.4 E Center protruding Spherical crown shape 40 39.3 F Center protruding Spherical crown shape 45 9.8 G Recessed — — 2.5 Total 100

In Table 6, in A to F type dimples, all depths d of the center protruding portions are values of depths d shown in Table 1, but the diameters of the dimples (diameters of substantially hexagonal circumscribed circles) differ according to the radii of curvature R of the protruding portions. The A type dimple has the smallest size, and the size of the dimple increases in order from the B type dimple to the F type dimple. The G type dimple is a conventional dimple formed of a support pin.

The golf ball of Example 1 having such a configuration was subjected to tests to evaluate the amount of spin and flight distance of the golf ball. First, a driver club (“TourB XD-5” (W #1) (loft angle: 9.5°) manufactured by Bridgestone Sports Co., Ltd.) was mounted to a golf ball hitting robot, and a golf ball as a sample was hit at a head speed of 45 m/s to measure the amount of backspin and the flight distance. A middle iron club (“TourB X-CB” (I #6) manufactured by Bridgestone Sports Co., Ltd.) was mounted to a golf ball hitting robot, and a golf ball as a sample was hit at a head speed of 42 m/s to measure the amount of backspin. The results are shown in Table 1.

For comparison, golf balls of Comparative Examples 2 to 4 were produced. The golf balls were shown in Table 2 and had the same configuration as that of Example 1 except that the material hardness of a cover, the volume occupation ratio VR of a dimple, and the depth d of the center protruding portion of the dimple were appropriately changed from those of Example 1 so as not to satisfy the condition of Formula 1. The golf balls were subjected to tests to evaluate the amounts of spin and flight distances of the golf balls in the same manner as in Example 1. The results are shown in Table 2. The dimples of Comparative Examples 2 to 4 also have the same specifications as those of Table 6. In each example, the depth d of the center protruding portion was changed without the diameters of the A to G type dimples being changed (Examples 2 to 6 and Comparative Example 1 to be described later are also the same).

As shown in Tables 1 and 2, the golf ball of Example 1 having a dimple volume occupation ratio VR of less than 0.75 and satisfying the condition of (H−78)/(−300)>d of Formula 1 had a dimple volume occupation ratio VR of less than 0.75, but had a less amount of backspin in a driver shot than that of Comparative Examples 2 and 3 not satisfying the condition of Formula 1, which could have an increased flight distance. The golf ball of Example 1 also had a less amount of backspin in the middle iron than that of Comparative Examples 2 and 3. This is also considered to make it possible to increase the flight distance in the middle iron.

Furthermore, the golf ball of Example 1 had substantially the same amount of backspin in the driver shot as that of Comparative Example 4 satisfying the condition of Formula 1 and having a dimple volume occupation ratio VR of 0.75 or more, but the golf ball of Example 1 having a volume occupation ratio VR of 0.70 could have an increased flight distance. The golf ball of Example 1 had substantially the same amount of backspin in the middle iron as that of Comparative Example 4. Therefore, the golf ball of Example 1 having a volume occupation ratio VR of 0.70 is also considered to make it possible to increase the flight distance in the middle iron.

Golf balls of Examples 2 and 3 were produced. The golf balls were shown in Table 1 and had the same configuration as that of Example 1, except that the material hardnesses of covers were the same as those of Comparative Examples 2 and 3, and the volume occupation ratio VR of a dimple and the depth d of the center protruding portion of the dimple were changed so as to satisfy the condition of Formula 1. The golf balls were subjected to tests to evaluate the amounts of spin and flight distances of the golf balls in the same manner as in Example 1. The results are shown in Table 1.

As shown in Tables 1 and 2, the golf balls of Examples 2 and 3 had a lesser amount of backspin in a driver shot than that of Comparative Examples 2 and 3 not satisfying the condition of Formula 1, which could have an increased flight distance. The golf balls of Examples 2 and 3 also had a lesser amount of backspin in the middle iron than that of Comparative Examples 2 and 3. This is also considered to make it possible to increase the flight distance in the middle iron.

Table 2 shows the golf ball of Comparative Example 1 having the same configuration as that of Example 1, except that the golf ball of Comparative Example 1 has the same material hardness of the cover as that of Example 1, and the volume occupation ratio VR of a dimple and the depth d of the center protruding portion of the dimple were changed so as not to satisfy the condition of Formula 1. In a simulation considering the above test results, in Comparative Example 1, the depth d of the protruding portion is too great as shown in Table 2, whereby the contact area of the golf ball with the club is small, and the golf ball of Comparative Example 1 had an amount of backspin in a driver shot greater than, and a flight distance less than, those of Example 1. The golf ball of Comparative Example 1 is also considered to have a greater amount of backspin in the middle iron than that in Example 1, which has a decreased flight distance.

A golf ball of Comparative Example 5 was produced. The golf ball had the same configuration as that of Example 1, except that the apex of a center protruding portion 35 had a flat shape, as illustrated in FIGS. 3 and 4 , whereas the depth d of the center protruding portion of a dimple was maintained at the same depth as that of Example 1. More specifically, a bottom 34 shaped from one end of an outer periphery 32 of the dimple 30 to the other end has a center protruding portion 35 with a curved shape protruding toward the outside of the ball in a center region thereof, but has an apex having a flat shape. A distance W between both ends 37 of the flat region is equal to a distance W between both ends 17 of a spherical crown shape region illustrated in FIG. 2 . The depth d of the flat region of the center protruding portion 35 is also determined on the basis of the line S connecting both ends of the outer periphery 32 of the dimple as the reference as with FIG. 2 . In regions on both sides of the center protruding portion 35, the bottom of the dimple is curved so that the depth D thereof becomes greatest at a deepest point 38.

The more detailed specifications of the dimples are shown in Table 7. The H to M type dimples in Table 7 respectively correspond to the A to F type dimples in Table 6, and have the same dimple diameter. The N type dimple is a conventional dimple formed of a support pin. The N type dimple is subjected to tests to evaluate the amount of spin and the flight distance as with Example 1. The results are shown in Table 2.

TABLE 7 Shape of Radius of Ratio of Type of Shape of Protruding curvature dimple dimple bottom portion R (mm) (%) H Center protruding Flat — 3.7 I Center protruding Flat — 5.5 J Center protruding Flat — 9.8 K Center protruding Flat — 29.4 L Center protruding Flat — 39.3 M Center protruding Flat — 9.8 N Recessed — — 2.5 Total 100

The photograph of a dimple when the golf ball of Example 1 is discharged at the rate of 35 m/s to be caused to collide with a transparent plate is shown in FIG. 5 . The photograph of dimples when a golf ball of Comparative Example 5 is caused to collide with a transparent plate under the same condition is shown in FIG. 6 .

As shown in FIG. 5 , in the golf ball of Example 1 in which the center protruding portion of the dimple has a spherical crown shape, almost the entire surface of the protruding portion contacts the transparent plate, whereas as shown in FIG. 6 , in the golf ball of Comparative Example 5 in which the center protruding portion of the dimple is flat, the center place of the protruding portion does not sufficiently contact the transparent plate. Therefore, as shown in Table 2, the golf ball of Comparative Example 5 satisfies Formula 1 which is the relationship between the material hardness of the cover and the depth d of the protruding portion, and has a dimple volume occupation ratio VR of less than 0.75, but has a smaller contact area of the golf ball with the club face in full shots than that of Example 1, whereby the golf ball of Comparative Example 5 had an amount of backspin in a driver shot greater than, and a flight distance less than, those of Example 1. The golf ball of Comparative Example 5 is also considered to have a greater amount of backspin in the middle iron than that in Example 1, which has a decreased flight distance.

Furthermore, as shown in Table 8, golf balls of Reference Examples 1 and 2 were produced. The golf balls had dimples having the same flat protruding portion as that of Comparative Example 5, except that delustering particles were blended with a paint layer, the condition of Formula 1 was not satisfied, and the volume occupation ratio VR of the dimple was set to be 0.68 and 0.75. The golf balls were subjected to tests to evaluate the amount of spin and the flight distance as with Example 1.

TABLE 8 Reference Examples 1 2 Intermediate layer Blending A A Material hardness 57 57 Cover Blending C C Material hardness H 55 55 Paint layer Blending G G Delustering particles Present Present Dimples Number 326 326 VR 0.68 0.75 Protruding Number 318 318 Shape of apex Plane Plane Depth d [mm] 0.085 0.085 Value of (H-78)/(−300) 0.077 0.077 Value of (H-78)/(−300)−d −0.008 −0.008 Evaluation of Formula 1 Not satisfied Not satisfied Amount of spin (driver) Evaluation 2738 2742 [rpm] Poor Poor Flight distance (driver) Evaluation 232.1 228.5 [m] Good Bad Amount of spin (middle iron) Evaluation 5131 5122 [rpm] Good Good

The amounts of backspin in a driver shot in the golf balls of Reference Examples 1 and 2 with which the delustering particles were blended were slightly greater than that of Comparative Example 5, but the golf ball of Reference Example 1 having a decreased volume occupation ratio VR of 0.68 had an increased flight distance, whereas the golf ball of Reference Example 2 having an increased volume occupation ratio VR of 0.75 had a decreased flight distance. It is considered that the amounts of backspin in the middle iron in the golf balls of Reference Examples 1 and 2 are also almost equal to each other, but the golf ball of Reference Example 1 having a decreased volume occupation ratio VR of 0.68 has an increased flight distance, and the golf ball of Reference Example 2 having an increased volume occupation ratio VR of 0.75 has a decreased flight distance.

Table 1 shows golf balls of Examples 4 and 5 having the same configuration as that of Example 1, except that delustering particles were blended with a paint layer, and the volume occupation ratio VR of a dimple and the depth d of the center protruding portion of the dimple were appropriately changed. In a simulation considering the above test results, as shown in Table 1, the amounts of backspin in a driver shot in the golf balls of Examples 4 and 5 were the almost equal to each other, but the golf ball of Example 4 having a volume occupation ratio VR of 0.68 can have an increased flight distance as compared with the golf ball of Example 5 having the same volume occupation ratio VR of 0.72 as that of Example 1. The amounts of backspin in the middle iron in the golf balls of Examples 4 and 5 were also almost equal to each other. Therefore, an increase in the flight distance in the middle iron of the golf ball of Example 4 is also considered to be superior to that of Example 5.

Table 1 shows the golf ball of Example 6 having the same configuration as that of Example 1, except that the material hardness of the intermediate layer is less than the material hardness of the cover. In a simulation considering the above test results, as shown in Table 1, the amount of backspin in the driver shot in the golf ball of Example 6 is equal to that in Example 1, and the flight distance can be maintained, but the amount of backspin in the middle iron is increased. Therefore, the increase in the flight distance in the middle iron is considered to be poor. 

What is claimed is:
 1. A golf ball comprising a core, and a cover located outside the core and having a plurality of dimples on a surface thereof, wherein a bottom of each of the dimples has a protruding portion having a curved shape protruding toward an outside of the golf ball; the protruding portion has a spherical crown shape having a radius of curvature R of 20 to 50 mm; a depth d of the protruding portion of the bottom of the dimple is a perpendicular distance between a line S connecting both ends of an outer periphery of the dimple and a highest point of the protruding portion; a volume occupation ratio VR of the dimple is less than 0.75; and a relationship between Shore D hardness H of a material of the cover and the depth d (unit: mm) satisfies the following Formula 1: (H−78)/(−300)>d  (Formula 1).
 2. The golf ball according to claim 1, wherein the Shore D hardness of the material of the cover is 50 to
 60. 3. The golf ball according to claim 1, further comprising an intermediate layer between the core and the cover, wherein Shore D hardness of a material of the intermediate layer is 55 or more.
 4. The golf ball according to claim 3, wherein the Shore D hardness of the material of the intermediate layer is higher than the Shore D hardness of the material of the cover.
 5. The golf ball according to claim 1, further comprising a coating layer located outside the cover, wherein: the coating layer contains delustering particles; average roughness Ra of a surface of the coating layer is 0.5 to 1.0; and the volume occupation ratio VR of the dimple is less than 0.70.
 6. The golf ball according to claim 1, wherein the number of dimples having the protruding portion having the spherical crown shape having the radius of curvature R of 20 to 50 mm is 50% or more of the total number of dimples on a surface of the cover.
 7. The golf ball according to claim 1, wherein in addition to the dimples having the protruding portion having the spherical crown shape having the radius of curvature R of 20 to 50 mm, dimples having a protruding portion having a spherical crown shape having a radius of curvature R of 12 mm or less are arranged, and the number of the dimples having the protruding portion having the spherical crown shape having the radius of curvature R of 12 mm or less is 1 to 10% of the total number of dimples on a surface of the cover. 